NEW YORK (CNNMoney.com) -- Banks continue to write off credit card debt as consumers hurt by record high unemployment default at an increasing rate.

Regulatory forms filed this week by some of the nation's largest banks showed default rates on credit cards rose in May. The default rate is a measure of loans that the bank does not expect to be repaid.

"Data from May showed continued signs of stress for card issuers, reflective of worsening unemployment trends and deteriorating macro [economic] conditions," analysts at Bernstein Research said in a report Tuesday.

Bank of America (BAC, Fortune 500), the nation's largest bank, said its default rate jumped to 12.5% in May from 10.5% the month before. Other major banks, including Citigroup (C, Fortune 500), JPMorgan Chase (JPM, Fortune 500) and Capital One (COF, Fortune 500), also reported increases in May default rates.

However, delinquency rates, which reflect the number of borrowers who are at least 30 days late on a payment, decreases slightly in May.

According to Bernstein Research, the average 30-day delinquency rate decreased in May to 1.57% from 1.71% the month before. It was the second month in a row that 30-day delinquency rates declined.

Delinquency rates are considered an indicator of future credit losses, but analysts say default rates will probably remain high until a recovery in the labor market takes hold.

"While this could clearly be a positive development, as we should now be past the typical seasonal improvements, it's too early to call it a sustainable trend, particularly in light of the deteriorating unemployment picture," Bernstein analysts wrote.

The unemployment rate rose to a 26-year high in May, even as the number of jobs lost in the month decreased significantly, and analysts say the job market will remain weak for some time after the overall economy recovers.

At the same time, analysts said the decrease in May delinquencies could be due to consumers taking advantage of one-time economic stimulus benefits, such as income tax cuts, to pay down debt.

"The wild card is really unemployment, which is the main driver of delinquencies and defaults," said Chris Deritis, an economist at Moody's Economy.com.